A pebble mill is a type of ball mill characterized by a cylindrical or conical vessel rotated on a horizontal axis. Pebble mills can have steel inner walls, but often are lined with ceramic, rubber, plastic or other materials. Pebble mills are used in conjunction with milling media that are typically steel or ceramic, although other milling media can be used.
Pebble mills can be in the form of cylindrical jars which are loaded from a sealable port on one end of the jar. During the milling process, smaller pebble mills are often placed on a powered set of rollers. Larger pebble mills often consist of a cylindrical vessel, mounted horizontally on pins along its longitudinal axis and driven by a shaft, gears, or belts. Such pebble mills commonly further comprise a fixed shroud which encloses the milling vessel. The shroud allows for discharging of the mill while the vessel is being rotated. Pebble mill vessels sometimes have double walls to allow for water cooling of the milling vessel, lifters to prevent the slippage of the media on the interior walls of the vessel, and/or ports to allow gas purging during operation.
Pebble mills are commonly used to grind powders to fine particle sizes or to disperse powders or pigments in solvents. In conventional dry grinding operations pebble mills are typically filled to half of their volume with a mixture of the milling media and the powder to be ground (commonly termed as the millbase). The milling media is commonly of three types: spherical, cylindrical or irregular. In the case of spherical milling media, the volume ratio of the milling media to the millbase is typically 30:20. The rotational speed of the vessel is set so that the media inside the vessel form a continuous cascade, with the cascade angle being in a range of 45-60° with respect to horizontal. These conditions are widely used as they are optimal for the efficient milling of powders. At higher rotational speeds the media tend to be launched into the air inside the vessel. At even higher rotational speeds the media can become pinned to the sides of the vessel by centrifugal force. The theoretical rotational speed in revolutions per minute (rpm) at which the media becomes pinned to the vessel's side is called the critical speed of the mill and is given by:rpmcritical=54.2/R0.5 where R is the inside diameter of the milling vessel expressed in feet.